John Bahcall, Richard Black Professor of Astrophysics at the Institute for Advanced Study at Princeton, had a long and distinguished career in astronomy and astrophysics which spanned five decades and during which he published more than 500 technical papers, books and popular articles.
Probably his most recognised scientific contribution was his longstanding involvement with the physics of neutrinos from the sun. From the beginning, the solar neutrino problem illustrated his genius for bringing experiment and theory together. Neutrinos are elementary particles that travel at nearly the speed of light. They were known to be produced by many of the nuclear reactions going on in the sun, but they interact very, very weakly with matter and are very difficult to detect.
In 1964 Bahcall, together with the experimentalist Raymond Davis Jnr, made a novel double-barrelled proposal for a way to detect the neutrinos coming to us from the sun and to show that a quantitative measurement of just how many arrive would solve various mysteries about the sun. In particular, one would get information on just how hot the sun is, which of various nuclear reactions make the most neutrinos and, indirectly, even how old the sun is.
Bahcall had made theoretical predictions, based on the most standard models for the interior structure of the sun and standard particle physics models. In the 1960s and 1970s Davis managed to observe the flux of neutrinos actually received at the earth in an experiment in a deep mine which involved chlorine as a detector. There was a clear discrepancy between measurement and theory, which led to a great controversy.
Either the theoretical models for the interior of the sun, which includes the temperature at the centre of the sun, had to be wrong - even though the fundamental physics was known - or the basic theory of elementary particles had to be wrong. The former would be merely embarrassing, since it would mean some mistake in a calculation, whereas the latter would be exciting, leading to new fundamental physics. Bahcall, in typical fashion, showed how quite different neutrino detectors, in addition to Davis's, could resolve the discrepancy.
Over the last 10 to 15 years such large-scale experiments were indeed carried out in Japan, Canada, Russia and Italy and we now have the answer: the solar models are correct but particle physics has to be modified. In particular, neutrinos must have a mass and oscillate between different particle states, as Bahcall had already surmised. This led, deservedly, to the 2002 Nobel Prize to Raymond Davis Jnr and the Japanese team leader, Masatoshi Koshiba. I personally feel that Bahcall deserved also to be included, but at any rate his enormous contribution is appreciated by all physicists and astronomers.
John Norris Bahcall was born in 1934 in Shreveport, Louisiana, received his AB from the University of California, Berkeley, in 1957 and his PhD from Harvard University in 1961. For five years from 1965, he was on the faculty at Caltech (the California Institute of Technology) in Pasadena, where he was influenced by various luminaries but especially interacted with the nuclear experimental physicist Willy Fowler.
Hans Bethe at Cornell University had already shown which nuclear reactions produce the energy in the sun and in other "main sequence" (i.e. unevolved) stars in a 1939 theoretical paper. However, what reactions go on in highly evolved stars, or went on in the first few minutes after the expansion which we call the Big Bang, was not so clear.
To get a start on these problems, many nuclear physics experiments using nuclei accelerated to high energy were required. Willy Fowler at Caltech did many such experiments in the 1950s and 1960s, which were quite definitive, but to get real answers one also needed many theoretical calculations. Bethe acted as a role model for this endeavour and Fred Hoyle, then at Cambridge University, played an important role, but younger theorists were also required and John Bahcall was one of the most prominent of these on quite a number of topics. This collective effort of theorists helped to make nuclear astrophysics into a science of its own (and got Willy Fowler a Nobel Prize later on).
In 1971 Bahcall was appointed to the faculty at the Institute for Advanced Study and had remained there ever since. He created the astronomy group there, which became a leading training ground for young postgraduate researchers. As a counterpoint to the introduction of theory into nuclear astrophysics at Caltech, he was instrumental in fostering an appreciation of experimental physics and of astronomical telescope observations at the IAS.
Long before Bahcall, the IAS was famous for very brilliant mathematicians and abstract theoretical physicists such as J. Robert Oppenheimer, John von Neumann and Albert Einstein. Without actually building laboratories at the IAS, Bahcall almost single-handedly trained theorists there to make their calculations dovetail in with experiments and observations. More generally, he managed to erase the somewhat "élitist" reputation of the IAS by inviting and training young scientists and then finding suitable jobs for them. Without Bahcall, the question of "who got Einstein's office" might be an important one at the IAS, but with him the relevant questions were more egalitarian.
Bahcall also helped in building up astronomy groups at the Weizman Institute and Tel Aviv University in Israel. He was a powerful driving force in the scientific community in the United States, especially leading the effort in the 1970s (with Lyman Spitzer) to create the Hubble Space Telescope and chairing the committee which created the decade roadmap for US astronomy research. The committee recommendations came to be known as the Bahcall Report and were listened to over a considerable period .
Aside from the value of his researches, Bahcall was an example of how to combine a light touch with reliability. I myself always enjoyed writing joint papers with him: he started work with refreshing originality and ended up with meticulous, quantitative detail.
He was a recipient of the National Medal of Science (from President Bill Clinton) in 1998, was President of the American Astronomical Society and President-elect of the American Physical Society (APS). Other honours included the Hans Bethe Prize (APS), the Dan David Prize of Israel and the Gold Medal of the Royal Astronomical Society; and he shared with others the Fermi Award and the Benjamin Franklin Medal in Physics.
John Bahcall is survived by his wife Neta, also a professor of astrophysics, and by three grown-up children who are also academics (of whom their daughter, Orli, studied in England and is with the scientific journal Nature Genetics).
Edwin E. Salpeter